What is a Blocking Diode?

By controlling the current flow direction, diodes can be used be used to accomplish different functions in electronic devices. Diodes are available in different types, each specifically made to achieve certain functionalities in circuits. For example, to achieve high-speed switching functionality in circuits, tunnel diodes are usually used; they are highly doped making the depletion layer smaller in size hence electrons move across the junction with minimal voltage making them the preferred choice in fast switching applications. To achieve voltage regulation in circuits, a zener diode used; it is a special kind of diode designed to exploit the reverse breakdown voltage.

To prevent current from flowing in a given direction in a circuit, blocking diodes are used; in this article I will explain the blocking diode principles, the common types, applications and the design considerations for engineers as well as hobbyists designing power systems.

Diode Basic Principles

What is a Diode?

A diode is a two-terminal semiconductor device that acts as a one-way gate to electric current flow i.e. it allows current to flow in only one direction when forward biased and blocks current flow when reverse biased.

When a diode’s anode lead is made more positive in voltage than its cathode, we create a condition referred to as forward biasing – current is permitted to flow through the device. But if we reverse the polarities, that is, the anode is made more negative in voltage than the cathode; we create a condition referred to as reverse biasing. The diode acts to block current flow.

In forward bias, the diode presents a forward voltage drop (V_f) and a forward current (I_f). In reverse bias, it allows only a small leakage current until the reverse breakdown voltage (V_r or V_RRM) is reached, after which large current flows, which can cause damage to the device.  

Common semiconductor materials used to fabricate diodes include silicon and germanium. Both offer high resistance to current flow in one direction (reverse bias) and allow current with ease in the opposite direction (forward bias).

What is a Blocking Diode?

A blocking diode is a unidirectional semiconductor device normally placed in a power path to prevent the current from flowing in the undesired direction. It allows the current flow from a source to a load while blocking reverse current that could for example, discharge a battery, flow back into a power source, or damage components in an electrical system

Blocking Diode Concept

A blocking diode is usually placed in a circuit so that forward conduction allows intended power flow, (for example, from charger to battery) and reverse conduction is blocked (e.g. from battery to charger when charger is off). 

Just like any other typical p-n junction diode, a blocking diode has two terminals, anode and cathode. If the anode is connected to a positive voltage and the cathode to a negative voltage, the diode is said to be forward biased. If we reverse the polarity, the diode is in reverse biased condition, and the current is blocked. This is the concept that makes blocking diodes useful for preventing the reverse currents in power supply systems.

The blocking diode’s forward voltage drop and leakage current determines how much voltage and power are lost and whether the blocking action is effective for the given application.

Key Electrical Parameters for Blocking Diodes

Key parameters to consider when selecting blocking diodes include:

• Forward voltage (V_f) – the forward voltage drop when conducting, it depends on the diode type and current.
• Maximum forward current (I_F(AV)) – the continuous current the diode can safely carry. The diode’s rating must be higher than the maximum continuous current the power supply will output.
• Reverse repetitive peak voltage (V_RRM or V_r), the maximum reverse voltage the diode can block. This must exceed the highest possible voltage from the load, including a significant safety margin of about 25% to 50%.
• Reverse leakage current (I_R) – this is the small current that flows in reverse bias. An important parameter in low power systems.
• Reverse recovery time (t_rr) – the amount of time it takes the diode to turn off when the voltage across it alternates from forward-bias to reverse-bias polarity. This parameter matters in switching applications.
• Junction temperature (T_J) limits and thermal resistance (ƟJA, ƟJC).

Effects of Reverse Currents in a Circuit

What is reverse current?

Reverse current is the current that flows opposite the intended direction in a circuit, that is, from load back into the source or from one source into another. It arises when a pathway allows the source to be driven by the load or by a different source with a higher potential in the reverse direction.

Main causes of reverse current include:

• Wrong polarity or wiring.
• The back-feed between multiple power sources e.g. battery into charger or into another battery.
• Components reverse conduction (diode breakdown, MOSFET body diode conduction when V_sd or V_gs conditions allow).
• Leakage through components (e.g. diode reverse leakage, MOSFET body diode, resistors, etc.).
• Inductive kickback or transients forcing current in reverse during switching events.
• Parasitic discharge through the supply’s internal bleeder resistor when the output is disabled i.e. the resistor continues to draw the current even after the supply output has been disabled, which can drain say a battery connected to the system which must never fall below a minimum cell voltage.
• Reverse current from the unregulated source surge e.g. a battery pack during relay connection. If a power supply is first connected to a battery pack, we may have a momentary voltage mismatch at the instant of connection. If the battery pack voltage is higher than the supply’s setpoint, the battery pack becomes the source, forcing surge of reverse current into the supply. This inrush reverse current can have detrimental effect on components.

What are the Effects of Reverse Current in Circuits?

Reverse current can have detrimental effect on internal circuits and powers supplies such as batteries. Additionally, it can damage cables and connectors and in worst situations can cause the device to fail or explode.

All in all reverse current can have the following effects on circuits:

• Battery discharge – discharges batteries when a source is off, reducing runtime and causing unintended deep discharge.
• Energy loss and heating – reverse current through series elements such as diodes, resistors, MOSFETs causes power dissipation (P = V x I or I²R), heating and reduced efficiency.
• Damage to sources or components: reverse currents can overstress chargers, power supplies, or sensitive circuits; it can trigger reverse-voltage breakdown in diodes and semiconductors.
• False operation or malfunction – back-fed supply rails may power portions of a system unexpectedly, causing logic errors, incorrect sequencing, or failure to reset.
• Safety hazards – unwanted currents can bypass protections such as fuses and switches and create hot spots or drive hazardous conditions.
• The increased leakage and self-discharge – in low-power systems, small reverse currents (microamps to milliamps) significantly shorten the long-term battery life. 
• Unequal current sharing particularly in current sources tied without proper isolation, reverse currents can cause one source to sink current from another, stressing the weaker or lower-voltage sources.

Mitigation Techniques for Reverse Currents in Circuits

To alleviate the issue of reverse currents, here are the possible mitigation methods:

• Blocking diodes or ideal-diode (MOSFET) controllers to prevent back-flow.
• Proper OR-ing or power-path controllers for multiple sources.
• Series switches or relays to isolate sources when unused.
• Low-leakage components for standby-sensitive designs.
• Fuses or current-limiting to protect against fault reverse currents.
• TVS diodes, snubbers, and RC networks to limit transients.
• Lastly, employ appropriate design practices – apply correct wiring, sequencing, and margining of voltages to avoid unintended back-feed.

Common Types used as Blocking Diodes

Blocking diodes in circuits can be implemented with the following types:

Standard Silicon Diodes

These are characterized by a forward voltage drop V_f of about 0.6 to 1.0 V at moderate currents.

They are robust, inexpensive, and widely available.

Their higher forward voltage drop means that we have higher power loss and heat for high-current applications.

Schottky Diodes

Schottky diodes have lower forward voltage drop (V_f) lower than silicon at the same current (V_f ≈ 0.15 – 0.6 V).

They are characterized by faster switching, lower forward drop, and higher leakage especially at elevated temperatures.

Schottky diodes are usually preferred for low-voltage systems and where efficiency matters.

Related article: Schottky Diode – Features & Applications

Fast-Recovery and SiC Diodes

Fast-recovery silicon or SiC diodes offer low switching losses and high blocking voltages.

SiC diodes have higher V_r capability and lower switching loss; V_f can be higher depending on the device.

They are useful in high-voltage or high-temperature systems.

MOSFET-based Ideal-Diode Controllers and FETs

The blocking diode function is achieved by using a MOSFET configured to conduct with a very low Rds(on) in forward direction and block in reverse using body diode control or synchronous switching.

They are characterized by low voltage drop and power loss compared to discrete diodes.

They have a slightly higher complexity and cost, but preferred for efficiency-critical or high-current systems.

Comparison of the Various Types

• Silicon diodes are cheap, robust but with higher forward voltage drop Vf.
• Schottky diodes have lower V_f but higher leakage.
• SiC/fast-recovery diodes have high-voltage/high-temperature use.
• MOSFET ideal diode has the lowest loss and higher complexity.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         

Generally standard silicon rectifiers are robust and ideal for high-voltage diode applications, whereas Schottky diodes, with their much lower forward voltage drop (of about 0.2 V to 0.4 V) are better for lower-voltage, high-current applications where minimizing heat is paramount.

Typical Applications of Blocking Diodes

Solar PV Systems

This is the most common use of blocking diodes where blocking diodes are used in off-grid solar systems that charge batteries.

This solar PV system operation incorporating blocking diodes can be described as follows:

During the day, the solar panel’s voltage is usually higher than the battery’s voltage, so the current flows naturally from the panel into the battery. But at night, the panel stops generating power and its voltage drops to almost zero, whilst the battery remains charged. Since the battery is at higher voltage potential, the current would want to flow backward, from the battery into the panel. The blocking diode is used, to block the reverse current from the battery to the solar panel; otherwise the reverse current flow can drain the battery charge.

The blocking diode is usually installed in series between the solar panel and the battery as illustrated in the figure below:

The blocking diodes provide unidirectional current flow from the solar panel to the battery, and block currents from the battery to the solar cells during the night as the solar cells during this period act as a load instead of generating power.

Battery Charging Systems

A blocking diode is used between the charger and battery to prevent battery discharge into the charger when the charger is off. In solar charge controllers, it prevents backflow at night.

Reverse Polarity Protection

A blocking diode placed in series with the supply prevents damage if polarity is reversed.

Low-side vs. high-side placement choices affect ground referencing and sensing.

Power OR-ing/Source Selection

Diode-OR permits multiple power sources to feed a load while preventing back feeding between the sources such as battery and mains supply.

This works by allowing the highest voltage source to supply the load whilst the diode blocks the other power sources. 

Load Isolation and Emergency Cutoffs

Diodes are used to isolate the subsystems so that one’s fault does not back-feed another. They can be combined with fuses and switches to enhance safety.

Design Considerations for Blocking Diodes Applications

Selecting Diode Type

• For applications that require low-voltage drop and higher efficiency, use Schottky or MOSFET-based solutions.
• For high-voltage, high-temperature, or high reliability applications, you may consider SiC or fast-recovery diodes.
• For very low standby loss, MOSFET ideal-diode arrangements are optimal.

Current Rating and Derating

Select diodes with I_F greater than the expected continuous current plus margin (typical 20 – 50 % derating). Consider surge and inrush currents (peak repetitive surge current, I_FSM).

Reverse Voltage Rating

Choose V_RRM at least 1.5 x the maximum system voltage, and consider more if transients are expected.

Power Dissipation and Thermal Management

Power loss is given by P = V_f x I_f  (estimated). For MOSFETs, P ≈ I² x Rds(on).

Calculate the expected dissipated power, and then use the package thermal resistance to estimate junction temperature. Where necessary provide robust thermal management – heat sinks or PCB copper.

Leakage Effects in Low-Power Systems

The reverse leakage current from Schottky diodes can be significant at elevated temperature; in a battery trickle or low-standby systems, leakage may drain the battery over long periods. Use low-leakage components or MOSFE switching where necessary.

Transient Behavior and Surge Protection

Diodes can be exposed to transient overvoltages or surge currents during operation. Use TVS diodes, snubbers, and other appropriate surge-rated diodes when required.

Consider reverse recovery in switching environments which may cause electromagnetic interference.

Efficiency Impact and Voltage Drop Calculations

Always account for the forward voltage drop V_f when you are sizing power supplies and the battery charging thresholds. Take the case in point, of a 12 V system; a 0.6 V drop across the blocking diode reduces the battery charging voltage available at the cell by 5%, this may prevent the full charge.

Blocking Diodes vs. Bypass Diodes

Blocking diodes can be thought of ‘one-way gates’ that stop reverse flow of current, and bypass diodes as ‘detour routes’ that go around broken sections. Bypass diodes are usually connected externally in parallel with the photovoltaic cells in reverse bias to provide an alternative path for current flow in case there is a problem with photovoltaic cells such as a broken path or damaged cells. Common diodes used as bypass diodes include the pn-junction diode and Schottky diode.

Here are how the two types of diodes compare in terms of their features, functionality and applications:

• Function – locking diodes prevent reverse current i.e. battery draining at night while bypass diodes prevent hot spots (damaged cells overheating).
• Location – blocking diodes are in series on the main positive line (between panel and battery/charge controller). Bypass diodes are placed in parallel across sub-strings of solar cells inside the junction box.
• Impact on Voltage – blocking diodes cause a small voltage drop typically 0.3 – 0.7 V, reducing power slightly. On the other hand, bypass diodes preserve system voltage by isolating only the bad section, so that the rest of the panel keeps working.
• Failure mode – if a blocking diode fails open, the system stops charging. If it fails short, the battery drains at night. If a bypass diode fails open, that panel section overheats and can catch fire. If it fails short, that panel string is permanently bypassed (loses power).
• When required – blocking diodes are essential for lead-acid batteries and PWM controllers, but unnecessary for most MPPT controllers (they have built-in protection). Bypass diodes are always essential for all modern solar panels.

Conclusion

The blocking diode property of allowing current flow from the source to the load and blocking when reverse biased is helpful in power systems as it prevents backflow which can be harmful to circuit elements if allowed to happen.

Blocking diode functionality in power systems can be accomplished using the standard silicon diodes, Schottky diodes, silicon carbide (SiC) and fast recovery diodes, MOSFET-based ideal diode controllers, etc. Each application type is chosen in such a way to achieve the design specification of a given power system.                                                         

While the concept of blocking diodes may not be that too hard for anyone to understand, complexity may come in when you are designing a system that incorporates a blocking diode and you are required to make the right choices with regard to key parameters, bearing in mind that any erroneous selection of a diode type for a given application or incorrect sizing can affect the efficiency or have a detrimental effect on the power supply, the load and all the connecting components. So understanding the various ways to achieve blocking diode functionality in a power system helps you avoid errors in design and ensure you build reliable systems.                                   

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• The Ultimate Guide to Diodes on Circuit Boards
• PIN Diode – Features & Applications